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I was surfing around the Rambo/Ramps pages on another thing and came across this:

http://reprap.org/wiki/RAMPS#Safety_Tip_2

Get a smoke detector. Especially if you've got long prints.....

Nice reminder. I might just stick a battery powered detector right onto the top plate for earliest possbile detection.

Maybe it would be prudent to have the detector trip a relay that would kill power to the whole printer

Edit: thinking about this further, one could wire in a 110v smoke detector. Most have the capability of alerting all detectors in the house via a 3wire cable: two wires are hot/neutral and the third is a switch leg to alert other detectors on the circuit. You could use that third wire to power a normally closed relay that cuts power to the printer. This would actually be pretty easy to implement.

Who needs a smoke detector when you have this little guy Although I do have a detector right over my MAX